The present invention relates to a 3D (three-dimensional) ultrasonic diagnostic apparatus which visualizes a 3D region within a human body under examination and more specifically to a technique for improving the real-time imaging.
This application is based on Japanese Patent Application No. 10-311367 filed on Oct. 30, 1998 and Japanese Patent Application No. 10-316584 filed on Nov. 6, 1998, the entire content of which is incorporated herein by reference.
Recent 2D types of ultrasonic probes can scan a 3D region within a human body under examination with ultrasound to produce a 3D image. This type of scanning is referred to as 3D scanning or volume scanning. Whereas conventional 2D scanning is required only to move ultrasound along a plane section of a human body, the 3D scanning needs to move ultrasound in all directions within a 3D region of the human body. In order to reproduce natural movements of internal organs in real time, it is required to reduce the time required to scan the 3D region thoroughly for the purpose of improving temporal resolution (volume rate). That is, it is required to set the number of times that the 3D region is scanned every second to about 30 times per second as in the 2D scanning.
As is well known, the velocity of propagation of ultrasound through human body is nearly constant; therefore, the number of times per unit time that ultrasound is transmitted and received is limited. That is, since the time required for transmission and reception of an ultrasound beam is absolutely determined by the depth of field and the ultrasound propagation velocity, the transmission/reception rate is almost fixed.
In order to satisfy the real-time requirements of the 3D scanning, therefore, it is required to reduce the spatial resolution (the density of ultrasound scanning lines). In order to increase the number of ultrasound scanning lines per second, the adoption of a simultaneous reception scheme known as digital beam forming has been considered. However, even with the digital beam forming, echoes are only received from some directions at most for each transmission, resulting in a failure to gain sufficient spatial resolution. It might be expected to increase the spatial resolution by increasing the number of directions from which echoes are received simultaneously. However, this approach would require applied energy to be considerably high and therefore might cause damage to the array probe and fail to meet safety standards.
The ultrasonic 3D imaging method, as its typical operation, extracts concerned parts from 2D image data obtained, and superimposes the extracted concerned parts one on another to create a 3D image. In this method, therefore, part of 2D image data drops off.
Further, it is very useful in diagnosis to display a tissue image (B-mode image) and a blood-flow image (color Doppler image) in combination. However, the 3D representation capability is still being improved.
With the ultrasonic imaging, although its imaging range is narrower than the imaging range of X-ray computerized tomography apparatus and magnetic-resonance imaging apparatus, . . . This causes a problem in that it is difficult for an observer to understand the orientation and position of a 3D image in a human body under examination.